The trailing end point phenotype in antifungal susceptibility testing is pH dependent

Impact of pH on the antifungal susceptibility of vaginal Candida albicans

OBJECTIVE

To investigate the antifungal susceptibility at pH 7.0 and pH 4.0 of 5 antifungal agents against Candida albicans isolated from patients with vulvovaginal candidiasis.

METHODS

Antifungal susceptibility testing at pH 7.0 and pH 4.0 was performed using the broth microdilution method (CLSI, document M27-A2).

RESULTS

The minimal inhibitory concentrations (MICs) of miconazole, clotrimazole, fluconazole, and nystatin against C. albicans at pH 4.0 were significantly higher than those at pH 7.0 (0.25 vs 0.03 μg/mL, 0.50 vs 0.03 μg/mL, 0.50 vs 0.25 μg/mL, and 32 vs 2 μg/mL, respectively; P<0.001), whereas the MIC of itraconazole at pH 4.0 was lower than that at pH 7.0 (0.030 vs 0.125 μg/mL; P<0.001). The susceptibility rate of C. albicans to itraconazole at pH 4.0 was significantly higher than at pH 7.0 (95.0% vs 51.7%; P<0.001). The susceptibility rate to itraconazole at pH 7.0 was significantly lower than the susceptibility rate to fluconazole (51.7% vs 100.0%; P<0.001), whereas the susceptibility rates to the 2 drugs were similar at pH 4.0 (95.0% and 96.7%, respectively).

CONCLUSION

Media at different pH values should be used for sensitivity tests according to the environment of C. albicans.

T-2307 shows efficacy in a murine model of Candida glabrata infection despite in vitro trailing growth phenomena

T-2307, a novel arylamidine, has been shown to exhibit broad-spectrum in vitro and in vivo antifungal activities against clinically significant pathogens. In our preliminary studies, Candida glabrata exhibited significant trailing growth (partial inhibition of growth over an extended range of antifungal concentrations) in the presence of T-2307 when it was tested using the Clinical and Laboratory Standards Institute (CLSI) guidelines with 0.2% glucose and 48 h of incubation, making reading of the MIC difficult. In the present study, we attempted to attenuate trailing growth to avoid misreading of the MIC. On the basis of the hypothesis that T-2307 may inhibit the mitochondrial functions of cells, the carbon source or the glucose concentration in the medium was changed. The trailing growth of C. glabrata ATCC 90030 in the presence of T-2307 was attenuated as the concentration of glucose in the medium decreased to 0.1% or lower, and trailing growth was completely inhibited when glycerol was used. A susceptibility test using Alamar blue was performed to facilitate reading of the MIC without changing the composition of the medium and provided a clear MIC endpoint at 24 h. To investigate if T-2307 shows efficacy against trailing isolates in vivo, we evaluated the efficacy of T-2307 in a murine model of disseminated candidiasis caused by C. glabrata. T-2307 at 0.05 mg/kg of body weight/day significantly decreased the viable count in the kidneys compared to that for the control group (P < 0.05). It would be better to test the susceptibility of C. glabrata to T-2307 using modified media or Alamar blue to avoid misreading of the MIC due to the significant trailing growth.

Mutants of Candida albicans hypersensitive to calcofluor white display susceptibility to antifungal drugs

An increased infection incidence of Candida albicans (most common human fungal pathogen) contributes to the need of further functional genetic studies and development of new antifungal drugs. We developed a method to create mutants of C. albicans using an antisense cDNA library to interfere with gene expression, followed by screening for hypersensitivity to Calcofluor White (CFW) and the antifungal drugs caspofungin and itraconazole. Mutants with these properties have with a high probability defects in cell-wall integrity. Fifty out of 200 transformant colonies analyzed (25%) showed hypersensitivity to CFW compared with the parental strain C. albicans CAI-4. Most of those CFW-hypersensitive mutants further displayed the susceptibility to antifungal drugs itraconazole and caspofungin using microbroth dilution method M27-A and an agar-diffusion test. The mutants obtained through this procedure could provide a potential model for screening antifungal pro-drugs which show weak action when standard C. albicans strain is used and may also aid in further identifying genes involved in cell integrity. In addition, we describe the effect of varying several parameters in electroporation transformation, including treatment with lithium acetate, upon the efficiency of transformation in C. albicans.

Mechanism of the synergistic effect of amiodarone and fluconazole in Candida albicans

The antiarrhythmic drug amiodarone has been found to have fungicidal activity. In Saccharomyces cerevisiae, its antifungal activity is mediated by calcium overload stress, which leads to a rapid nuclear accumulation of the calcineurin-regulated transcription factor CRZ1. In addition, low doses of amiodarone have been reported to be synergistic with fluconazole in fluconazole-resistant Candida albicans. To establish its mechanism of toxicity in C. albicans, we used expression profiling of key pathway genes to examine cellular responses to amiodarone alone and in combination with fluconazole. Gene expression profiling of 59 genes was done in five C. albicans strains (three fluconazole-susceptible strains and two fluconazole-resistant strains) after amiodarone and/or fluconazole exposure. Of the 59 genes, 27 analyzed showed a significant change (>2-fold) in expression levels after amiodarone exposure. The up- or downregulated genes included genes involved in Ca(2+) homeostasis, cell wall synthesis, vacuolar/lysosomal transport, diverse pathway regulation, stress response, and pseudohyphal morphogenesis. As expected, fluconazole induces an increase in ergosterol pathway genes expression levels. The combination treatment significantly dampened the transcriptional response to either drug, suggesting that synergism was due to an inhibition of compensatory response pathways. This dampening resulted in a decrease in total ergosterol levels and decreased pseudohyphal formation, a finding consistent with decreased virulence in a murine candidiasis model.

Trailing or paradoxical growth of Candida albicans when exposed to caspofungin is not associated with microsatellite genotypes

Multilocus microsatellite polymorphisms in 27 clinical Candida albicans isolates were found to be clearly unrelated to in vitro paradoxical growth or trailing effect with caspofungin. These findings suggest that such in vitro phenotypes are either gained or lost too rapidly to be predicted by more stable genomic markers.

[A study for testing the antifungal susceptibility of yeast by the Japanese Society for Medical Mycology (JSMM) method. The proposal of the modified JSMM method 2009]

The Japanese Society for Medical Mycology (JSMM) method used for testing the antifungal susceptibility of yeast, the MIC end point for azole antifungal agents, is currently set at IC(80). It was recently shown, however that there is an inconsistency in the MIC value between the JSMM method and the CLSI M27-A2 (CLSI) method, in which the end- point was to read as IC(50). To resolve this discrepancy and reassess the JSMM method, the MIC for three azoles, fluconazole, itraconazole and voriconazole were compared to 5 strains of each of the following Candida species: C. albicans, C. glabrata, C. tropicalis, C. parapsilosis and C. krusei, for a total of 25 comparisons, using the JSMM method, a modified JSMM method, and the CLSI method. The results showed that when the MIC end- point criterion of the JSMM method was changed from IC(80) to IC(50) (the modified JSMM method) , the MIC value was consistent and compatible with the CLSI method. Finally, it should be emphasized that the JSMM method, using a spectrophotometer for MIC measurement, was superior in both stability and reproducibility, as compared to the CLSI method in which growth was assessed by visual observation.

Comparison of 24-hour and 48-hour voriconazole MICs as determined by the Clinical and Laboratory Standards Institute broth microdilution method (M27-A3 document) in three laboratories: results obtained with 2,162 clinical isolates of Candida spp. and other yeasts

We evaluated the performance of the 24-h broth microdilution voriconazole MIC by obtaining MICs for 2,162 clinical isolates of Candida spp. and other yeasts; the 24-h results were compared to 48-h reference MICs to assess essential, as well as categorical, agreement. Although the overall essential agreement was 88.6%, it ranged from 96.4 to 100% for 6 of the 11 species or groups of yeasts tested. The overall categorical agreement was 93.2%, and it was above 90% for eight species. However, unacceptable percentages of very major errors (false susceptibility) were observed for Candida albicans (2.7%), C. glabrata (4.1%), C. tropicalis (9.7%), and other less common yeast species (9.8%). Since it is essential to identify potentially resistant isolates and breakpoints are based on 48-h MICs, it appears that the 24-h MIC is not as clinically useful as the 48-h reference MIC. However, further characterization of these falsely susceptible MICs for three of the four common Candida spp. is needed to understand whether these errors are due to trailing misinterpretation or if the 48-h incubation is required to detect voriconazole resistance. Either in vivo versus in vitro correlations or the determination of resistance mechanisms should be investigated.

Lack of correlation of 24- vs. 48-h itraconazole minimum inhibitory concentrations with microbiological and survival outcomes in a guinea pig model of disseminated candidiasis

A 'trailing' effect has been commonly observed when azole antifungals are tested against Candida spp. Previous experience with fluconazole indicates that 24-h minimum inhibitory concentration (MIC) values are more compatible endpoints when compared with clinical outcomes. We evaluated the trailing effect of Candida isolates tested with itraconazole in a guinea pig model of systemic candidiasis. Survival and organ burden were only significantly affected by using a higher dose of itraconazole, irrespective of the MIC differences at 24 and 48 h. A fluconazole-resistant strain with susceptible dose-dependent MICs to itraconazole was successfully treated with high-dose itraconazole. Our data suggests that survival and microbiological response depend more on drug dosing than on the trailing phenotype of the isolates.

Comparison between the standardized clinical and laboratory standards institute M38-A2 method and a 2,3-Bis(2-Methoxy-4-Nitro-5-[(Sulphenylamino)Carbonyl]-2H-tetrazolium hydroxide- based method for testing antifungal susceptibility of dermatophytes

In this study, we determined the utility of a 2,3-bis(2-methoxy-4-nitro-5-[(sulfenylamino)carbonyl]-2H-tetrazolium hydroxide (XTT)-based assay for determining antifungal susceptibilities of dermatophytes to terbinafine, ciclopirox, and voriconazole in comparison to the Clinical and Laboratory Standards Institute (CLSI) M38-A2 method. Forty-eight dermatophyte isolates, including Trichophyton rubrum (n = 15), Trichophyton mentagrophytes (n = 7), Trichophyton tonsurans (n = 11), and Epidermophyton floccosum (n = 13), and two quality control strains, were tested. In the XTT-based method, MICs were determined spectrophotometrically at 490 nm after addition of XTT and menadione. For the CLSI method, the MICs were determined visually. With T. rubrum, the XTT assay revealed MIC ranges of 0.004 to >64 mug/ml, 0.125 to 0.25 mug/ml, and 0.008 to 0.025 mug/ml for terbinafine, ciclopirox, and voriconazole, respectively. Similar MIC ranges were obtained against T. rubrum by using the CLSI method. Additionally, when tested with T. mentagrophytes, T. tonsurans, and E. floccosum isolates, the XTT and CLSI methods resulted in comparable MIC ranges. Both methods revealed similar lowest drug concentrations that inhibited 90% of the isolates for the majority of tested drug-dermatophyte combinations. The levels of agreement within 1 dilution between both methods were as follows: 100% with terbinafine, 97.8% with ciclopirox, and 89.1% with voriconazole. However, the agreement within 2 dilutions between these two methods was 100% for all tested drugs. Our results revealed that the XTT assay can be a useful tool for antifungal susceptibility testing of dermatophytes.

Characterization of caspofungin susceptibilities by broth and agar in Candida albicans clinical isolates with characterized mechanisms of azole resistance

Caspofungin (CSP) susceptibilities of Candida albicans, as determined by broth microdilution methods, have not been found to be related to azole susceptibilities or resistance. In contrast, it has been observed that azole-resistant clinical isolates that overexpress the efflux pump gene CDR2 are less susceptible to CSP when tested using an agar dilution method commonly employed with Saccharomyces cerevisiae. The goal of this study was to further understand the effects of azole resistance mechanisms on CSP susceptibility testing. A collection of 69 isolates exhibiting known mechanisms of azole resistance and resistance-associated phenotypes were analyzed by broth microdilution methods to determine standard minimum inhibitory concentrations (MICs) for CSP. The same isolates were then analyzed as to their MIC to CSP by Etest strips, an agar-based method that has been shown generally to be comparable to broth methods. The MICs found with both methods were not significantly different. However, a collection of strains overexpressing the efflux pump CDR2 did exhibit a spectrum of CSP susceptibilities when examined by agar dilution susceptibility tests, ranging from standard to reduced susceptibilities. This work demonstrated that a change in CSP susceptibility with CDR2 overexpressing cells in agar dilution studies is a variable phenotype and it is not the result of growth conditions (i.e., broth versus agar).

Micafungin activity against Candida albicans with diverse azole resistance phenotypes

OBJECTIVES

The purpose of this study was to investigate whether mechanisms of azole resistance in Candida albicans contribute to reduced micafungin activity in vitro.

METHODS

MICs were determined for a collection of strains with well-characterized mechanisms of azole resistance obtained from systemic, oral and vaginal infections. This collection of strains includes those with resistance-associated phenotypes. All known molecular mechanisms of azole resistance are included in this set of isolates (alone or in combination). Micafungin activity was further investigated for a subset of isolates by agar dilution.

RESULTS

There was no correlation between any of the azole resistance mechanisms or resistance phenotypes and micafungin activity as determined by MIC, even in isolates with cross-resistance to multiple azole drugs. Overexpression of the ABC transporter CDR2 has been suggested to contribute to reduced echinocandin activity in agar dilution studies. By broth microdilution, there was no difference in MIC between the pump overexpressors and the collection as a whole. However, azole-resistant isolates from matched strains exhibited a small increase in their micafungin MICs relative to their susceptible controls. By agar dilution analysis, multiple CDR2-overexpressing strains exhibited reduced growth in the presence of micafungin relative to the laboratory strain SC5314.

CONCLUSIONS

Azole resistance mechanisms do not contribute to increased micafungin MIC as determined by broth microdilution. However, within sets of matched isolates, strains overexpressing CDR2 had a slight increase in micafungin MIC. Changes in micafungin susceptibility are associated with CDR2 overexpression in agar dilution tests.

Current status of antifungal susceptibility testing methods

Antifungal susceptibility testing is a very dynamic field of medical mycology. Standardization of in vitro susceptibility tests by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee for Antimicrobial Susceptibility Testing (EUCAST), and current availability of reference methods constituted the major remarkable steps in the field. Based on the established minimum inhibitory concentration (MIC) breakpoints, it is now possible to determine the susceptibilities of Candida strains to fluconazole, itraconazole, voriconazole, and flucytosine. Moreover, utility of fluconazole antifungal susceptibility tests as an adjunct in optimizing treatment of candidiasis has now been validated. While the MIC breakpoints and clinical significance of susceptibility testing for the remaining fungi and antifungal drugs remain yet unclear, modifications of the available methods as well as other methodologies are being intensively studied to overcome the present drawbacks and limitations. Among the other methods under investigation are Etest, colorimetric microdilution, agar dilution, determination of fungicidal activity, flow cytometry, and ergosterol quantitation. Etest offers the advantage of practical application and favorable agreement rates with the reference methods that are frequently above acceptable limits. However, MIC breakpoints for Etest remain to be evaluated and established. Development of commercially available, standardized colorimetric panels that are based on CLSI method parameters has added more to the antifungal susceptibility testing armamentarium. Flow cytometry, on the other hand, appears to offer rapid susceptibility testing but requires specified equipment and further evaluation for reproducibility and standardization. Ergosterol quantitation is another novel approach, which appears potentially beneficial particularly in discrimination of azole-resistant isolates from heavy trailers. The method is yet investigational and requires to be further studied. Developments in methodology and applications of antifungal susceptibility testing will hopefully provide enhanced utility in clinical guidance of antifungal therapy. However, and particularly in immunosuppressed host, in vitro susceptibility is and will remain only one of several factors that influence clinical outcome.

Paradoxical growth effects of the echinocandins caspofungin and micafungin, but not of anidulafungin, on clinical isolates of Candida albicans and C. dubliniensis

OBJECTIVES

To analyze the effects of a high concentration of three antifungal substances, the echinocandins anidulafungin, caspofungin, and micafungin, on the growth of Candida spp.

METHODS

The growth of 127 C. dubliniensis isolates and 103 C. albicans isolates cultured in medium containing anidulafungin, caspofungin, or micafungin was analyzed using a broth microdilution test according to the guidelines of the CLSI M27-A2 [NCCLS (1997), Wayne, PA]. The final concentrations of all three echinocandins ranged from 0.125 to 64 microg/L.

RESULTS

The different effects of these three antifungal substances on C. albicans cells in comparison to C. dubliniensis cells were quite distinct. When both Candida species were grown in the presence of anidulafungin only a trailing effect was observed. Micafungin induced an Eagle effect in C. dubliniensis only (63%), while caspofungin induced this effect in the majority of C. dubliniensis isolates (90%) and in only a few C. albicans isolates (14%).

CONCLUSIONS

Based on our observations, anidulafungin has effects that are different from the ones produced by micafungin and caspofungin. Whether this different response to high concentrations of echinocandins is based on genetic or phenotypic differences between C. albicans and C. dubliniensis has to be determined in future experiments.

Discovery of uracil-based histone deacetylase inhibitors able to reduce acquired antifungal resistance and trailing growth in Candida albicans

Among fungal pathogens such as Candida albicans, acquired drug resistance has not been associated with plasmids or other transferable elements, but it is thought to involve primarily mutations and genetic or epigenetic phenomena. This prompted us to test some histone deacetylase inhibitors (HDACi) from our library, in combination with fluconazole, against C. albicans strains in vitro. Among the tested compounds, the two chloro-containing uracil-hydroxamates 1c and 1d showed a strong reduction of the MIC values on Candida strains that show the trailing growth effect. In this assay, 1c,d were more potent than SAHA, a well-known HDAC inhibitor, in reducing the Candida growth. More interestingly, 1c,d as well as SAHA were able to inhibit the fluconazole-induced resistance induction in Candida cultures.

In vitro activity of panomycocin, a novel exo-?-1,3-glucanase isolated from Pichia anomala NCYC 434, against dermatophytes

Killer proteins that are produced and secreted into the environment by certain yeast strains are considered as promising antifungal agents. In this study, in vitro activity of Pichia anomala NCYC 434 (K5) killer protein, panomycocin, which is a 49 kDa glycoprotein with an exo-beta-1,3-glucanase activity was tested against 41 isolates of dermatophytes. Minimum inhibitory concentrations (MICs) were determined by a broth microdilution method based on the reference document M38-A of Clinical and Laboratory Standards Institute (CLSI; formerly NCCLS). For panomycocin MIC determinations two end point criteria MIC-2 (prominent growth inhibition) and MIC-0 (complete growth inhibition) were recorded. All the tested isolates (Microsporum spp. and Trichophyton spp.) were found susceptible to panomycocin. The MIC-2 values ranged from 0.25 to 2 microg ml(-1) and MIC-0 values ranged from 1 to 8 microg ml(-1). These results showed that panomycocin is active in vitro against fungal strains that cause superficial infections and highlighted its probable use as a topical antifungal agent.

Identification of Candida species isolated from patients in intensive care unit and in vitro susceptibility to fluconazole for a 3-year period

Species level identification of Candida and antifungal susceptibility testing is not generally performed in routine laboratory practice. There is limited information about the distribution of Candida species and antifungal susceptibility in Turkey. In this study, we aimed at identifying Candida isolates to species level from various samples obtained from patients treated in an intensive care unit between 2002 and 2005 and to evaluate fluconazole susceptibilities of the isolates. A total of 320 Candida isolates obtained from 270 patients were identified by conventional methods and using API (Candida and/or 20C AUX) system. Antifungal susceptibility testing was performed by broth microdilution method. Candida albicans was isolated with the highest frequency (65.6%) followed by C. parapsilosis (11.3%), C. glabrata (8.8%) and C. tropicalis (7.8%). Of all the isolates, 92.9% revealed susceptibility to fluconazole. Susceptibility to fluconazole was highest for C. albicans followed by C. parapsilosis and C. glabrata. The MIC(90) values for C. albicans, C. parapsilosis, C. glabrata and C. tropicalis were 1, 2, 8 and 4 mug ml(-1) respectively. Fluconazole remains effective against both C. albicans and the majority of non-albicans Candida species. In this study, we determine the distribution of Candida species and evaluate the susceptibilities of the isolates, particularly for the azoles.

In VitroActivities of Voriconazole and Five Licensed Antifungal Agents AgainstCandida dubliniensis:Comparison of CLSI M27-A2, Sensititre YeastOne, Disk Diffusion, and Etest Methods

We compared the in vitro activity of six antifungal agents against 62 isolates of Candida dubliniensis by the Clinical Laboratory Standards Institute (CLSI [formerly National Committee for the Clinical Laboratory Standards]) M27-A2, Sensititre YeastOne, disk diffusion, and Etest methods and we studied the effect of the time of reading. For the azoles, voriconazole was the most potent in vitro followed by fluconazole, ketoconazole, and itraconazole. All the isolates were susceptible to amphotericin B and flucytosine. The highest rate of resistance was obtained against itraconazole with a high number of isolates defined as susceptible dose-dependent. At 24 hr, 100% of the isolates were susceptible to ketoconazole, amphotericin B, and flucytosine, 98% susceptible to voriconazole and fluconazole, and 95% for itraconazole. At 48 hr, 100% of the isolates remained susceptible for flucytosine and amphotericin B, 95% for voriconazole, 93% for fluconazole, 90% for ketoconazole, and 82% for itraconazole. The agreement between the CLSI and the other methods was better at 24 than 48 hr.

Candida resistance and its clinical relevance

Since the 1970s, the death rate from Candida infection has risen in conjunction with increasing numbers of patients at risk for serious fungal infections, such as those immunocompromised because of tissue or organ transplantation, chemotherapy, acquired immunodeficiency syndrome, or advanced age. Candidal infections are not only prevalent but also associated with considerable mortality and morbidity. In 2005, the overall mortality rate was 44% within 30 days of the first blood culture positive for any Candida species. Given the substantial morbidity and mortality, clinicians must include fungal infection in the differential diagnosis for at-risk patients, and they must quickly select appropriate antifungal therapy for those with identified infection. However, clinicians cannot use the minimum inhibitory concentration to select antifungal therapy in the same way they use it to treat bacterial infections. The relationship between in vitro susceptibility and clinical effect is not as direct with antifungals as it is with antibiotics. As long as Candida species continue to be the major causes of fungal infections, improving outcomes remains an important therapeutic goal. A key element is preventing and managing drug resistance. Further study of treatment duration, dosage, intermittent-versus-continuous administration schedules, and other treatment options are needed to determine their effect on resistance. Therapy that combines agents with complementary mechanisms of action may increase potency and broaden the spectrum of antifungal efficacy while decreasing the number of resistant organisms.

Fluconazole treatment is effective against a Candida albicans erg3/erg3 mutant in vivo despite in vitro resistance

Candida albicans ERG3 encodes a sterol C5,6-desaturase which is essential for synthesis of ergosterol. Defective sterol C5,6 desaturation has been considered to be one of the azole resistance mechanisms in this species. However, the clinical relevance of this resistance mechanism is still unclear. In this study, we created a C. albicans erg3/erg3 mutant by the "Ura-blaster" method and confirmed the expected azole resistance using standard in vitro testing and the presence of ergosta-7,22-dien-3beta-ol instead of ergosterol. For in vivo studies, a wild-type URA3 was placed back into its native locus in the erg3 homozygote to avoid positional effects on URA3 expression. Defective hyphal formation of the erg3 homozygote was observed not only in vitro but in kidney tissues. A marked attenuation of virulence was shown by the longer survival and the lower kidney burdens of mice inoculated with the reconstituted Ura+ erg3 homozygote relative to the control. To assess fluconazole efficacy in a murine model of disseminated candidiasis, inoculum sizes of the control and the erg3 homozygote were chosen which provided a similar organ burden. Under these conditions, fluconazole was highly effective in reducing the organ burden in both groups. This study demonstrates that an ERG3 mutation causing inactivation of sterol C5,6-desaturase cannot confer fluconazole resistance in vivo by itself regardless of resistance measured by standard in vitro testing. The finding questions the clinical significance of this resistance mechanism.

An update on antifungal targets and mechanisms of resistance in Candida albicans

Much progress has been made in the last decade in identifying genes responsible for antifungal resistance in Candida albicans. Attention has focused on five major C. albicans genes: ABC transporter genes CDR1 and CDR2, major facilitator efflux gene MDR1, and ergosterol biosynthesis genes ERG11 and ERG3. Resistance involves mutations in 14C-lanosterol demethylase, targeted by fluconazole (FLZ) and encoded by ERG11, and mutations that up-regulate efflux genes that probably efflux the antifungals. Mutations that affect ERG3 mutations have been understudied as mechanism resistance among clinical isolates. In vitro resistance in clinical isolates typically involves step-wise mutations affecting more than one of these genes, and often unidentified genes. Different approaches are needed to identify these other genes. Very little is understood about reversible adaptive resistance of C. albicans despite its potential clinical significance; most clinical failures to control infections other than oropharyngeal candidiasis (OPC) occur with in vitro susceptible strains. Tolerance of C. albicans to azoles has been attributed to the calcineurin stress-response pathway, offering new potential targets for next generation antifungals. Recent studies have identified genes that regulate CDR1 or ERG genes. The focus of this review is C. albicans, although information on Saccharomyces cerevisiae or Candida glabrata is provided in areas in where Candida research is underdeveloped. With the completion of the C. albicans genomic sequence, and new methods for high throughput gene overexpression and disruption, rapid progress towards understanding the regulation of resistance, novel resistance mechanisms, and adaptive resistance is expected in the near future.

Anti-fungal activity of cathelicidins and their potential role in Candida albicans skin infection

Cathelicidins have broad anti-microbial capacity and are important for host defense against skin infections by some bacterial and viral pathogens. This study investigated the activity of cathelicidins against Candida albicans. The human cathelicidin LL-37, and mouse cathelicidin mCRAMP, killed C. albicans, but this fungicidal activity was dependent on culture conditions. Evaluation of the fungal membrane by fluorescent dye penetration after incubation with cathelicidins correlated membrane permeabilization and inhibition of fungal growth. Anti-fungal assays carried out in an ionic environment that mimicked human sweat and with the processed forms of cathelicidin such as are present in sweat found that the cleavage of LL-37 to forms such as RK-31 conferred additional activity against C. albicans. C. albicans also induced an increase in the expression of cathelicidin in mouse skin, but this induction did not confer systemic or subcutaneous resistance as mCRAMP-deficient mice were not more susceptible to C. albicans in blood-killing assays or in an intradermal infection model. Therefore, cathelicidins appear active against C. albicans, but may be most effective as a superficial barrier to infection.

Analysis of the influence of Tween concentration, inoculum size, assay medium, and reading time on susceptibility testing of Aspergillus spp

The influence of several test variables on susceptibility testing of Aspergillus spp. was assessed. A collection of 28 clinical isolates was tested against amphotericin B, itraconazole, voriconazole, and terbinafine. Inoculum size (10(4) CFU/ml versus 10(5) CFU/ml) and glucose supplementation (0.2% versus 2%) did not have significant effects on antifungal susceptibility testing results and higher inoculum size and glucose concentration did not falsely elevate MICs. In addition, antifungal susceptibility testing procedure with an inoculum size of 10(5) CFU/ml distinctly differentiated amphotericin B or itraconazole-resistant Aspergillus strains in vivo from the susceptible ones. Time of incubation significantly affected the final values of MICs, showing major increases (two to six twofold dilutions, P < 0.01 by analysis of variance) between MIC readings at 24 and 48 h, but no differences were observed between antifungal susceptibility testing results obtained at 48 h and at 72 h. Significantly higher MICs were uniformly associated with higher concentrations of Tween (P < 0.01), used as a dispersing agent in the preparation of inoculum suspensions. The geometric mean MICs showed increases of between 1.5- and 10-fold when the Tween concentration varied from 0.1% (the geometric means for amphotericin B, itraconazole, voriconazole, and terbinafine were 1.29, 0.69, 1.06, and 0.64 mug/ml, respectively) to 5% (the geometric means for amphotericin B, itraconazole, voriconazole, and terbinafine were 1.97, 5.79, 1.60, and 4.66 mug/ml, respectively). The inhibitory effect of Tween was clearly increased with inoculum sizes of 10(5) CFU/ml and was particularly dramatic for itraconazole, terbinafine, and Aspergillus terreus. The inoculum effect was not observed when the Tween concentration was below 0.5% (P > 0.01).

Altered susceptibility of Candida glabrata bloodstream isolates to triazoles at clinically relevant pH values: comparison of the NCCLS M27-A2, Sensititre YeastOne, and Etest methods

We studied the MICs of triazoles against 15 Candida glabrata clinical isolates by the NCCLS M27-A2, Sensititre YeastOne, and Etest methods by using media at pHs 6.0, 7.0, and 7.4. Thirteen isolates were less susceptible to triazoles at pH 6.0 and more susceptible to triazoles at pH 7.4 compared to pH 7.0.

Defined anaerobic growth medium for studying Candida albicans basic biology and resistance to eight antifungal drugs

The polymorphic fungus Candida albicans is one of the most versatile opportunistic pathogens in humans. Many organs of the human body are potential targets for infection by this pathogen, but infection is commonly localized in the gastrointestinal tract, an environment providing anaerobic growth conditions. We describe a chemically defined anaerobic growth medium for four strains of Candida albicans (A72, SC5314, MEN, and 10261). It is a defined liquid glucose-phosphate-proline growth medium supplemented with oleic acid, nicotinic acid, and ammonium chloride. The cells did not require or respond to added ergosterol. Oleic acid and nicotinic acid are growth factors which are required only for the anaerobic growth of C. albicans. An important technical feature of this study was the use of anaerobically grown inocula to study anaerobic growth. Anaerobically, the cells grew exclusively as mycelia at 25, 30, and 37 degrees C. The doubling time at 30 degrees C was ca. 20 h. The cells did not produce farnesol and did not respond to exogenous farnesol, and they were resistant to the highest tested levels of amphotericin B and four of the azole antifungals. We suggest that the anaerobic growth of C. albicans may contribute to the trailing end point phenomenon and the resistance of C. albicans biofilms to antifungal drugs.

Enhanced production of farnesol by Candida albicans treated with four azoles

The dimorphic fungus Candida albicans excretes farnesol, which is produced enzymatically from the sterol biosynthetic intermediate farnesyl pyrophosphate. Inhibition of C. albicans by four azole antifungals, fluconazole, ketoconazole, miconazole, and clotrimazole, caused elevated farnesol production (10- to 45-fold). Furthermore, farnesol production occurs in both laboratory strains and clinical isolates (J. M. Hornby et al., Appl. Environ. Microbiol. 67:2982-2992, 2001) of C. albicans.

Antifungal susceptibility survey of 2,000 bloodstream Candida isolates in the United States

Candida bloodstream isolates (n = 2,000) from two multicenter clinical trials carried out by the National Institute of Allergy and Infectious Diseases Mycoses Study Group between 1995 and 1999 were tested against amphotericin B (AMB), flucytosine (5FC), fluconazole (FLU), itraconazole (ITR), voriconazole (VOR), posaconazole (POS), caspofungin (CFG), micafungin (MFG), and anidulafungin (AFG) using the NCCLS M27-A2 microdilution method. All drugs were tested in the NCCLS-specified RPMI 1640 medium except for AMB, which was tested in antibiotic medium 3. A sample of isolates was also tested in RPMI 1640 supplemented to 2% glucose and by using the diluent polyethylene glycol (PEG) in lieu of dimethyl sulfoxide for those drugs insoluble in water. Glucose supplementation tended to elevate the MIC, whereas using PEG tended to decrease the MIC. Trailing growth occurred frequently with azoles. Isolates were generally susceptible to AMB, 5FC, and FLU. Rates of resistance to ITR approached 20%. Although no established interpretative breakpoints are available for the candins (CFG, MFG, and AFG) and the new azoles (VOR and POS), they all exhibited excellent antifungal activity, even for those strains resistant to the other aforementioned agents.

Fungicidal activity of fluconazole against Candida albicans in a synthetic vagina-simulative medium

Fluconazole (FLZ) has emerged as a highly successful agent in the management of systemic infections of Candida. Cure rates for symptomatic candidiasis following single 150-mg FLZ dose therapy exceed 90%. In vitro, however, FLZ is fungistatic only in a narrow pH range and is not effective at vaginal pH, 4.2. This study evaluated the effect of FLZ on Candida albicans under in vitro conditions resembling the vaginal microenvironment, using vagina-simulative medium (VS). We found that FLZ was fungicidal for C. albicans in VS, but not in other media at the same pH, 4.2. In VS, FLZ was fungicidal at concentrations of >/=8 micro g/ml and reduced viability by greater than 99.9%. Analysis of the components of VS indicated that 17 mM acetic acid, a concentration achieved in the vagina, was responsible for the synergistic, fungicidal effect. This effect was not seen at neutral pH. Other substrates were not effective substitutes for acetic acid; however, short-chained carboxylic acids, glyoxylate and malonate, were effective. Most strains of C. albicans that were resistant to FLZ under standard conditions were killed by FLZ plus acetate. Other species of Candida were also killed, except C. krusei and C. glabrata. This study shows that FLZ has fungicidal activity for Candida species under in vitro conditions that mimic the vaginal microenvironment. This raises the possibility that FLZ may also have fungicidal effects during treatment of vaginal candidiasis. Elucidating the mechanism by which FLZ and acetate interact may disclose vulnerable pathways that could be exploited in drug development.

Calcineurin A of Candida albicans: involvement in antifungal tolerance, cell morphogenesis and virulence

The azole antifungal fluconazole possesses only fungistatic activity in Candida albicans and, therefore, this human pathogen is tolerant to this agent. However, tolerance to fluconazole can be inhibited when C. albicans is exposed to fluconazole combined with the immunosuppressive drug cyclosporin A, which is known to inhibit calcineurin activity in yeast. A mutant lacking both alleles of a gene encoding the calcineurin A subunit (CNA) lost viability in the presence of fluconazole, thus making calcineurin essential for fluconazole tolerance. Consistent with this observation, tolerance to fluconazole was modulated by calcium ions or by the expression of a calcineurin A derivative autoactivated by the removal of its C-terminal inhibitory domain. Interestingly, CNA was also essential for tolerance to other antifungal agents (voriconazole, itraconazole, terbinafine, amorolfine) and to several other metabolic inhibitors (caffeine, brefeldin A, mycophenolic acid, fluphenazine) or cell wall-perturbing agents (SDS, calcofluor white, Congo red), thus indicating that the calcineurin pathway plays an important role in the survival of C. albicans in the presence of external growth inhibitors. Several genes, including PMC1, a vacuolar calcium P-type ATPase, were regulated in a calcineurin- and fluconazole-dependent manner. However, PMC1 did not play a direct role in the survival of C. albicans when exposed to fluconazole. In addition to these different properties, calcineurin was found to affect colony morphology in several media known to modulate the C. albicans dimorphic switch. In particular, calcineurin was found to be essential for C. albicans viability in serum-containing media. Finally, calcineurin was found to be necessary for the virulence of C. albicans in a mice model of infection, thus making calcineurin an important element for adequate adaptation to the conditions of the host environment.

Susceptibility testing of fungi--current status and open questions

The increase of fungal infections and the improvement of therapeutical options demand reliable antifungal susceptibility testing. In vitro susceptibility testing of fungi--in contrast to bacteria--is not yet established as a routine method. The NCCIS (National Committee for Clinical Laboratory Standards) guidelines for susceptibility testing of yeasts (and proposed for hyphomycetes) are most important for standardization. Meanwhile, essential parts of this test procedure are accepted, but it should still be improved. The concept of using only one test medium for all drugs and test organisms is not realized so far. There are also some test situations that prevent the NCCLS standard from being applied. Based on our experience, this article describes the NCCLS methods and their modifications. It places emphasis on lipophilic drugs showing controversies despite standardization. Furthermore, the prediction of MICs on the clinical outcome is discussed. Since there are some pitfalls in testing antifungals, this should be done in experienced laboratories only. The MIC has to be regarded as only one, but an important, factor in the management of fungal diseases. Host-, drug-, and pathogen-specific data should be considered simultaneously.

Mode of selection and experimental evolution of antifungal drug resistance in Saccharomyces cerevisiae

We show that mode of selection, degree of dominance of mutations, and ploidy are determining factors in the evolution of resistance to the antifungal drug fluconazole in yeast. In experiment 1, yeast populations were subjected to a stepwise increase in fluconazole concentration over 400 generations. Under this regimen, two mutations in the same two chromosomal regions rose to high frequency in parallel in three replicate populations. These mutations were semidominant and additive in their effect on resistance. The first of these mutations mapped to PDR1 and resulted in the overexpression of the ABC transporter genes PDR5 and SNQ2. These mutations had an unexpected pleiotropic effect of reducing the residual ability of the wild type to reproduce at the highest concentrations of fluconazole. In experiment 2, yeast populations were subjected to a single high concentration of fluconazole. Under this regimen, a single recessive mutation appeared in each of three replicate populations. In a genome-wide screen of approximately 4700 viable deletion strains, 13 were classified as resistant to fluconazole (ERG3, ERG6, YMR102C, YMR099C, YPL056C, ERG28, OSH1, SCS2, CKA2, SML1, YBR147W, YGR283C, and YLR407W). The mutations in experiment 2 all mapped to ERG3 and resulted in the overexpression of the gene encoding the drug target ERG11, but not PDR5 and SNQ2. Diploid hybrids from experiments 1 and 2 were less fit than the parents in the presence of fluconazole. In a variation of experiment 2, haploids showed a higher frequency of resistance than diploids, suggesting that degree of dominance and ploidy are important factors in the evolution of antifungal drug resistance.

Histone deacetylase inhibitors enhance Candida albicans sensitivity to azoles and related antifungals: correlation with reduction in CDR and ERG upregulation

Histone acetylation and deacetylation play important roles in eukaryotic gene regulation. Several histone deacetylase (HDA) inhibitors have been characterized, including trichostatin A (TSA), apicidin, and sodium butyrate. We tested their effects on Candida albicans in vitro growth, heat sensitivity, and germ tube formation; minimal effects were observed. However, there was a dramatic effect of TSA on C. albicans sensitivity to the azoles fluconazole, itraconazole, and miconazole. Similar effects were observed with other HDA inhibitors and with the antifungals terbinafine and fenpropimorph, which target, as do azoles, enzymes in the ergosterol biosynthetic pathway. In contrast, HDA inhibitors had minimal effect on the activities of amphotericin B, flucytosine, and echinocandin, which have unrelated targets. Specifically, addition of 3 micro g of TSA/ml lowered the itraconazole MIC for five susceptible C. albicans isolates an average of 2.7-fold at 24 h, but this increased to >200-fold at 48 h. Thus, the primary effect of TSA was a reduction in azole trailing. TSA also enhanced itraconazole activity against Candida parapsilosis and Candida tropicalis but had no effect with four less related yeast species. To examine the molecular basis for these effects, we studied expression of ERG genes (encoding azole and terbinafine targets) and CDR/MDR1 genes (encoding multidrug transporters) in C. albicans cells treated with fluconazole or terbinafine with or without TSA. Both antifungals induced to various levels the expression of ERG1, ERG11, CDR1, and CDR2; addition of TSA reduced this upregulation 50 to 100%. This most likely explains the inhibition of azole and terbinafine trailing by TSA and, more generally, provides evidence that trailing is mediated by upregulation of target enzymes and multidrug transporters.

Antifungal activity in Saccharomyces cerevisiae is modulated by calcium signalling

The most important group of antifungals is the azoles (e.g. miconazole), which act by inhibiting lanosterol demethylase in the sterol biosynthesis pathway. Azole activity can be modulated through structural changes in lanosterol demethylase, altered expression of its gene ERG11, alterations in other sterol biosynthesis enzymes or altered expression of multidrug transporters. We present evidence that azole activity versus Saccharomyces cerevisiae is also modulated by Ca2+-regulated signalling. (i) Azole activity was reduced by the addition of Ca2+. Conversely, azole activity was enhanced by the addition of Ca2+ chelator EGTA. (ii) Three structurally distinct inhibitors (fluphenazine, calmidazolium and a W-7 analogue) of the Ca2+-binding regulatory protein calmodulin enhanced azole activity. (iii) Two structurally distinct inhibitors (cyclosporin and FK506) of the Ca2+-calmodulin-regulated phosphatase calcineurin enhanced azole activity. (iv) Strains in which the Ca2+ binding sites of calmodulin were eliminated and strains in which the calcineurin subunit genes were disrupted demonstrated enhanced azole sensitivity; conversely, a mutant with constitutively activated calcineurin phosphatase demonstrated decreased azole sensitivity. (v) CRZ1/TCN1 encodes a transcription factor regulated by calcineurin phosphatase; its disruption enhanced azole sensitivity, whereas its overexpression decreased azole sensitivity. All the above treatments had comparable effects on the activity of terbinafine, an inhibitor of squalene epoxidase within the sterol biosynthesis pathway, but had little or no effect on the activity of drugs with unrelated targets. (vi) Treatment of S. cerevisiae with azole or terbinafine resulted in transcriptional upregulation of genes FKS2 and PMR1 known to be Ca2+ regulated. A model to explain the role of Ca2+-regulated signalling in azole/terbinafine tolerance is proposed.

Comparative evaluation of a new fluorescent carboxyfluorescein diacetate-modified microdilution method for antifungal susceptibility testing of Candida albicans isolates

This report presents a fluorescent carboxyfluorescein diacetate (CFDA)-modified microdilution method used for the susceptibility testing of Candida albicans to amphotericin B, fluconazole, ketoconazole, itraconazole, voriconazole, and flucytosine. Four different broth microdilution susceptibility testing methods were simultaneously evaluated at 24 and 48 h. The MICs determined using the CFDA-modified method (MIC(cfda)) were compared to those obtained by the standard broth microdilution method (MIC(visual)) and a procedure employing the indicator Alamar blue (MIC(alamar)). The reference MIC was determined visually as recommended by the NCCLS M27-A protocol, and then quantified spectrophotometrically following agitation (MIC(spec)). The CFDA-modified microdilution method was demonstrated to effectively determine the MICs for all the antifungal drugs tested at both 24 and 48 h. The results from both the MIC(spec) and MIC(cfda) methods yielded >80% agreement within +/-1 dilution and >90% agreement within +/-2 dilutions at 24 h in comparison to the reference MIC(visual) method, respectively. The trailing growth phenomenon that occurs with azole antifungal drugs and many strains of C. albicans did not inhibit the effectiveness of the MIC(spec) and MIC(cfda) methods. The MIC(spec) and MIC(cfda) methods shared 92.8% agreement within +/-1 dilution at 24 h and 87.6% agreement within +/-1 dilution at 48 h.

Resistance of Candida species to antifungal agents: molecular mechanisms and clinical consequences

Candida albicans and related species pathogenic for man become resistant to antifungal agents, in particular triazole compounds, by expression of efflux pumps that reduce drug accumulation, alteration of the structure or concentration of antifungal target proteins, and alteration of membrane sterol composition. The clinical consequences of antifungal resistance can be seen in treatment failures in patients and in changes in the prevalences of Candida species causing disease. These effects were seen unequivocally in HIV-infected patients with oropharyngeal candida infections, but their incidence has decreased dramatically with the introduction of highly active antiretroviral therapy. The evidence for similar emergence of antifungal-resistant yeast strains and species in other types of candida infections is confounded by non-standardised susceptibility testing methods and definitions of a resistant fungal isolate. Recent large-scale surveys of yeasts isolated from blood cultures, based on standardised methodology and resistance definitions, do not support the view that antifungal resistance in pathogenic yeasts constitutes a significant or growing therapeutic problem.

Present status of the detection of antifungal resistance: the perspective from both sides of the ocean

The NCCLS reference methodology for antifungal susceptibility testing is a new milestone of the evolution of medical mycology. The use of this methodology however, is not problem-free. At present, major limitations are a trailing phenomenon with azoles, unreliable detection of resistance to amphotericin B, poor growth of some organisms and unpractical procedures for the clinical laboratory. Herein a overview of NCCLS guidelines for yeasts and filamentous fungi is presented. Likewise, a review of studies conducted trying to overcome the limitations of reference procedures is also included. Several alternative approaches are reviewed as alternative media, inoculum size and incubation time. Modifications of reading procedure and endpoint determination are also evaluated. Agar diffusion methods and other methods for susceptibility testing are cited. Finally, we discuss the data on correlation of the in vitro results with the in vivo activity.

Antifungal susceptibility testing. New technology and clinical applications

The state of the art for susceptibility testing of yeasts is comparable with that of bacteria. Standardized methods for performing antifungal susceptibility testing are reproducible, accurate, and available in clinical laboratories. The development of quality control limits and interpretive criteria for a limited number of antifungal agents provides a basis for the application of this testing in the clinical laboratory. A proficiency testing program is available as a quality assurance measure for laboratories and has documented steady improvement among laboratories using the NCCLS method. As with antibacterial agents, surveillance programs are now in place using reference quality testing methods to monitor antifungal resistance trends on a global scale. It is clear that antifungal susceptibility testing can predict outcome in several clinical situations. Susceptibility testing is most helpful in dealing with infection caused by non-albicans species of Candida, and susceptibility testing of azoles is increasingly important in the management of candidiasis in critically ill patients. Susceptibility testing also has been standardized for filamentous fungi that cause invasive infections. Studies are ongoing to further refine this approach and evaluate the in vivo correlation with the in vitro data for molds. Future efforts must be directed toward establishing and validating interpretive break-points for licensed antifungals such as amphotericin B, and for new antifungals that are not yet licensed. Finally, procedures must be optimized for testing non-Candida yeasts (e.g., C. neoformans) and molds.

Antifungal susceptibility testing: practical aspects and current challenges

Development of standardized antifungal susceptibility testing methods has been the focus of intensive research for the last 15 years. Reference methods for yeasts (NCCLS M27-A) and molds (M38-P) are now available. The development of these methods provides researchers not only with standardized methods for testing but also with an understanding of the variables that affect interlaboratory reproducibility. With this knowledge, we have now moved into the phase of (i) demonstrating the clinical value (or lack thereof) of standardized methods, (ii) developing modifications to these reference methods that address specific problems, and (iii) developing reliable commercial test kits. Clinically relevant testing is now available for selected fungi and drugs: Candida spp. against fluconazole, itraconazole, flucytosine, and (perhaps) amphotericin B; Cryptococcus neoformans against (perhaps) fluconazole and amphotericin B; and Aspergillus spp. against (perhaps) itraconazole. Expanding the range of useful testing procedures is the current focus of research in this area.

In vitro antifungal susceptibility testing

With the rising frequency of fungal infections, as well as increasing reports of resistance to antifungal agents, it is imperative that clinically applicable antifungal susceptibility testing be available. In 1997 the National Committee for Clinical Laboratory Standards published standard guidelines for antifungal susceptibility testing of Candida sp and Cryptococcus neoformans with amphotericin B, flucytosine, fluconazole, itraconazole, and ketoconazole. Although the methods are standard, they are time consuming, can be difficult to interpret, and are approved only for testing limited organisms and drugs. Modifications to the methods and alternative approaches have been proposed to make these tests more convenient and efficient, applicable to a greater number of species, and appropriate for performing in the clinical laboratory.

Novel fluorescent broth microdilution method for fluconazole susceptibility testing of Candida albicans

A comparative evaluation of the reference National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution method with a novel fluorescent carboxyfluorescein diacetate (CFDA)-modified microdilution method for the susceptibility testing of fluconazole was conducted with 68 Candida strains, including 53 Candida albicans, 5 Candida tropicalis, 5 Candida glabrata, and 5 Candida parapsilosis strains. We found trailing endpoints and discordant fluconazole MICs of < 8 microg/ml at 24 h and of > or =64 microg/ml at 48 h for 12 of the C. albicans strains. These strains satisfy the definition of the low-high MIC phenotype. All 12 low-high phenotype strains were correctly shown to be susceptible at 48 h with the CFDA-modified microdilution method. For the 41 non-low-high phenotype C. albicans strains, the CFDA-modified microdilution method yielded 97.6% (40 of 41 strains) agreement within +/-1 dilution at 24 h compared with the reference method and 92.7% (38 of 41 strains) agreement within +/-1 dilution at 48 h compared with the reference method. The five strains each from C. tropicalis, C. glabrata, and C. parapsilosis that were tested showed 100% agreement within +/-2 dilutions for the two methods being evaluated.

Comparison of Etest and a tablet diffusion test with the NCCLS broth microdilution method for fluconazole and amphotericin B susceptibility testing of Candida isolates

Three methods were compared for the susceptibility testing of yeast isolates to fluconazole and amphotericin B: two fagar diffusion methods (Etest and a tablet diffusion test) and the National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution method. Given as MIC(50)s (range), fluconazole endpoints were: for the 24 h broth microdilution test, 0.25 mg/L (0.06-32 mg/L); for the Etest, 0.38 mg/L (0.064-24 mg/L); and for the NCCLS broth microdilution test, 2 mg/L (0.06->or=64 mg/L). With breakpoints of <3 mg/L for susceptible and >16 mg/L for resistant, the Etest and the 24 h microdilution test classified the isolates in agreement with the classification obtained by the NCCLS method. Results obtained by Etest were in closer NCCLS method than those obtained with the tablet test. Amphotericin B endpoints were lower for the 24 h microdilution and Etests than MICs obtained by the NCCLS broth microdilution method. Reproducibility was high for all tests; however, disadvantages of both diffusion tests were microcolonies in the inhibition zone and dependence on stringent standardization of inoculum.

Divergence in fitness and evolution of drug resistance in experimental populations of Candida albicans

The dissemination and persistence of drug-resistant organisms in nature depends on the relative fitness of sensitive and resistant genotypes. While resistant genotypes are expected to be at an advantage compared to less resistant genotypes in the presence of drug, resistance may incur a cost; resistant genotypes may be at a disadvantage in the absence of drug. We measured the fitness of replicate experimental populations of the pathogenic yeast Candida albicans founded from a single progenitor cell in a previous study (L. E. Cowen, D. Sanglard, D. Calabrese, C. Sirjusingh, J. B. Anderson, and L. M. Kohn, J. Bacteriol. 182:1515-1522, 2000) and evolved in the presence, and in the absence, of the antifungal agent fluconazole. Fitness was measured both in the presence and in the absence of fluconazole by placing each evolved population in direct competition with the drug-sensitive ancestor and measuring the reproductive output of each competitor in the mixture. Populations evolved in the presence of drug diverged in fitness. Any significant cost of resistance, indicated by reduced fitness in the absence of drug, was eliminated with further evolution. Populations evolved in the absence of drug showed more uniform increases in fitness under both conditions. Fitness in the competition assays was not predicted by measurements of the MICs, doubling times, or stationary-phase cell densities of the competitors in isolation, suggesting the importance of interactions between mixed genotypes in competitions.

Susceptibility testing of Aspergillus flavus: inoculum dependence with itraconazole and lack of correlation between susceptibility to amphotericin B in vitro and outcome in vivo

We have attempted to validate in Aspergillus flavus the main in vitro methodologies that have been used to detect resistance in Aspergillus fumigatus. We developed a murine model with two A. flavus isolates, one that was apparently resistant in vitro to amphotericin B (AFL5) and another that was resistant to itraconazole (AFL8). No correlation was found for amphotericin B in AFL5, since the in vivo response was compatible with a susceptible isolate. Modification of the in vitro susceptibility test methodology for amphotericin B was unsuccessful. Although AFL8 was apparently resistant to itraconazole in vitro, it was found to be susceptible in vivo. Additional in vitro work has detected weaknesses in the in vitro susceptibility methodology validated for A. fumigatus when applied to A. flavus. The principal problems are that changes in the inoculum have a large effect on the MICs of itraconazole for some A. flavus strains and that a trailing end point and spore sediment often appear when an inoculum with a higher colony count is used. We propose a modified method using a final inoculum of 2.5 x 10(4) CFU per ml of RPMI 1640 medium with 2% glucose buffered to pH 7.0 in a microtiter format, incubated for 48 h with no growth end point. Validation of this methodology requires one or more itraconazole-resistant A. flavus isolates, which have yet to be identified.

In vivo activity of amphotericin B lipid complex in immunocompromised mice against fluconazole-resistant or fluconazole-susceptible Candida tropicalis

We compared four doses of amphotericin B lipid complex (ABLC) with three doses of fluconazole in temporarily neutropenic mice in a murine model of disseminated candidiasis due to four different isolates of Candida tropicalis. The mice were infected with a 90% lethal dose of four strains of C. tropicalis for which the fluconazole MICs ranged from 1 to >125 mg/liter 3 days after receiving 200 mg of cyclophosphamide/kg of body weight. Treatment was started 18 h after infection and lasted for 7 days. ABLC (1, 2, 5, and 10 mg/kg) was administered once a day intravenously, fluconazole was administered by oral gavage once daily (25 and 50 mg/kg/day) or twice daily (125 mg/kg). MICs determined in five different ways with 24- and 48-h endpoints were also compared. The overall survival rates were controls, 14%; fluconazole, 64%; and ABLC, 82%. Treatment with ABLC at 2 to 10 mg/kg increased survival compared to controls (P = <0.0001) and was also superior to fluconazole at 25 and 50 mg/kg (P = 0.006). In the fluconazole-resistant C. tropicalis model (MIC, 128 microg/ml), ABLC at 2 to 10 mg/kg was superior to fluconazole at 250 mg/kg and ABLC at 10 mg/kg was superior to all fluconazole doses (P = <0.05). Fluconazole at 250 mg/kg daily was superior to both 25 and 50 mg/kg at reducing mortality with most isolates. ABLC was superior to fluconazole (P = <0.01), and fluconazole at 250 mg/kg was superior to fluconazole at both 25 and 50 mg/kg (P = 0.02) in all models at reducing C. tropicalis counts in the kidneys. Neither drug consistently sterilized the brain or kidneys. A 48-h endpoint reading with the NCCLS susceptibility testing microtiter variation overestimates resistance to fluconazole. ABLC is an effective treatment for fluconazole-resistant C. tropicalis at all doses tested.

Upregulation of ERG genes in Candida species by azoles and other sterol biosynthesis inhibitors

Infections due to Candida albicans are usually treated with azole antifungals such as fluconazole, but treatment failure is not uncommon especially in immunocompromised individuals. Relatedly, in vitro studies demonstrate that azoles are nonfungicidal, with continued growth at strain-dependent rates even at high azole concentrations. We hypothesized that upregulation of ERG11, which encodes the azole target enzyme lanosterol demethylase, contributes to this azole tolerance in Candida species. RNA analysis revealed that ERG11 expression in C. albicans is maximal during logarithmic-phase growth and decreases as the cells approach stationary phase. Incubation with fluconazole, however, resulted in a two- to fivefold increase in ERG11 RNA levels within 2 to 3 h, and this increase was followed by resumption of culture growth. ERG11 upregulation also occurred following treatment with other azoles (itraconazole, ketoconazole, clotrimazole, and miconazole) and was not dependent on the specific medium or pH. Within 1 h of drug removal ERG11 upregulation was reversed. Azole-dependent upregulation was not limited to ERG11: five of five ERG genes tested whose products function upstream and downstream of lanosterol demethylase in the sterol biosynthetic pathway were also upregulated. Similarly, ERG11 upregulation occurred following treatment of C. albicans cultures with terbinafine and fenpropimorph, which target other enzymes in the pathway. These data suggest a common mechanism for global ERG upregulation, e.g., in response to ergosterol depletion. Finally, azole-dependent ERG11 upregulation was demonstrated in three additional Candida species (C. tropicalis, C. glabrata, and C. krusei), indicating a conserved response to sterol biosynthesis inhibitors in opportunistic yeasts.